One-piece flywheel having outer ring gear portion, and process of manufacturing the same

ABSTRACT

A generally disc-shaped flywheel which is an integral one-piece forged structure formed by forging of a carbon steel blank and consisting of an inner portion having a friction surface and an outer ring gear portion disposed radially outwardly of the inner portion. The flywheel is formed by forging the carbon steel blank to obtain an intermediate product consisting of the inner portion and the outer ring gear portion, hardening the friction surface of the inner portion of the intermediate product, and shot peening the hardened friction surface to form a multiplicity of recesses in the hardened friction surface.

TECHNICAL FIELD

The present invention relates in general to a flywheel, and moreparticularly to improvements of a flywheel having an outer ring gearportion.

BACKGROUND ART

An engine such as an internal combustion engine operated by combustionof a fuel is widely used as a drive power source for automobiles, forexample. Such an engine inevitably suffers from a periodic variation inoperating speed (angular velocity) or torque in synchronization with acombustion cycle. To reduce or minimize this variation, a flywheel isusually attached to a crankshaft which is rotated by the engine. Aflywheel is also used to reduce a periodic variation in the operatingspeed of other rotating members not associated with an engine.

DE 44 03 306 discloses an example of a known two-piece flywheel used forautomobiles, which consists of an inner body and a separate ring gear.Another type of known two-piece flywheel is manufactured bypress-fitting or shrink-fitting an inner body into a separate ring gear,as indicated in FIG. 7. Generally, the ring gear is formed from a carbonsteel blank (e.g., S48C blank) usually used for machine structures, byhobbing or other machining operation using a pinion cutter, for example.The ring gear has a multiplicity of teeth formed along the periphery ofa ring member such that the teeth extend over the entire axial dimensionof the ring member, namely, ring gear. On the other hand, the inner bodyhas a central attaching section for attachment to a crankshaft, and isgenerally formed by casting of a gray iron such as FC230. The inner bodyis press-fitted or shrink-fitted into the opening of the ring gear toproduce the flywheel. That is, the flywheel consists of the inner bodymember and the outer ring gear which are fixed to each other.

Where the flywheel is used with an engine, the outer ring gear of theflywheel is held in meshing engagement with a pinion of an enginestarter motor, so that the crankshaft is rotated by the starter motor tostart the engine.

The known flywheel the inner body of which is formed of a cast gray irontends to suffer from insufficient tensile strength at its attachingsection. In view of this drawback, the attaching section isconventionally required to have a large wall thickness in the axialdirection of the flywheel. However, this solution results in an increasein the weight of the inner body member, and a decrease in the ratio ofthe weight of the outer ring gear to the total weight of the flywheel.Accordingly, the moment of inertia of the flywheel is reduced, leadingto an undesirable increase in the amount of variation in the rotarymotion of the crankshaft, and a consequent decrease in the fuel economyof the engine.

A further drawback of the known flywheel is a relatively high cost ofmanufacture, primarily because of the use of two blanks, which must beprocessed to produce the ring gear and the inner body member,respectively, before these two members are assembled together into theflywheel. Thus, there is a limitation in the reduction of themanufacturing cost of the known flywheel.

The conventional flywheel also has a problem of insufficient mechanicalstrength (insufficient toughness, for example) due to hardeningtreatment effected during the manufacture, while the flywheel isgenerally required to have a sufficiently high degree of durability at ahigh operating speed. In this respect, it is noted that the engine of amodem vintage is required to have a relatively high maximum operatingspeed.

DISCLOSURE OF INVENTION

It is therefore a first object of the present invention to provide aflywheel which has an increased strength at its outer ring gear portionand which is economical to manufacture.

It is a second object of this invention to provide a method ofeconomically manufacturing a flywheel having an increased strength atits outer ring gear portion.

The first object indicated above may be achieved according to a firstaspect of the present invention, which provides a flywheel which isgenerally disc-shaped and which includes a ring gear disposed along aperiphery thereof and has a friction surface on one of axially oppositesides thereof characterized in that the flywheel is an integralone-piece forged structure which is formed by forging a carbon steelblank and which consists of an inner portion having the friction surfaceand an outer ring gear portion which serves as the ring gear and whichis disposed radially outwardly of the inner portion.

The flywheel of the present invention constructed as described above isan integral one-piece forged structure formed by forging a carbon steelblank, for example, a blank of a carbon steel material for machinestructural use. Accordingly, the tensile strength of the flywheel at theattaching section of its inner portion is significantly increased, sothat the required wall thickness at the attaching section can beaccordingly reduced. Therefore, the present flywheel may be designed tohave an increased ratio of the weight of the outer ring gear portion tothe total weight, and an accordingly increased moment of inertia.

Further, the outer ring gear portion formed by forging has a higherstrength owing to a continuous metal structure, than the ring gear ofthe conventional flywheel, which is formed by hobbing or any other toothcutting operation.

In addition, the present one-piece flywheel can be manufactured at areduced cost, with a reduced number of process steps, since the integralone-piece forging structure of the flywheel is formed by forcing of thecarbon steel blank, without having to prepare two separate members, thatis, an inner body member and a ring gear member, and assemble these twomembers together, as required to manufacture the conventional flywheel.

In one preferred form of the present flywheel, the outer ring gearportion consists of a toothed section and a base section which are twomutually adjacent axial sections of the outer ring gear portion. Thetoothed section is formed by forging a cylindrical blank as the blank inan axial direction thereof and has a multiplicity of teeth which projectin a radially outward direction of the outer ring gear portion andextend in an axial direction of the outer ring gear portion and whichare spaced apart from each other in a circumferential direction of theouter ring gear portion. The teeth of the toothed section are connectedto each other by the base section which is adjacent to the toothedsection in the axial direction of the outer ring gear portion. In thisform of the flywheel, the toothed section of the outer ring gear portionhas an increased mechanical strength, in particular, increasedtoughness, even where the toothed section is hardened. Accordingly, theflywheel exhibits improved durability at a high operating speed.

The toothed section having the multiplicity of teeth is preferablylocated on the axial side of the flywheel which is remote from thefriction surface provided on the inner portion.

In another preferred form of the flywheel of the present invention, thefriction surface has a multiplicity of minute recesses or micro holesformed by shot peening after the forging on the carbon steel blank. Inoperation of the flywheel, the friction surface is brought intofrictional contact with a friction member such as a clutch disc. In thepresence of the minute recesses formed in the friction surface of theflywheel, there remains a network or matrix of air between the frictionsurface and the friction member, which makes it possible to minimize anincrease of the friction coefficient of the friction surface even at arelatively high operating temperature, thereby reducing the amount ofwear of the friction surface and prolonging the expected service life ofthe friction surface. In this respect, it is noted that the frictionalsliding contact of the friction surface and the friction member causes arise of the temperature at the friction surface. In the conventionalflywheel including the inner body member formed of a cast iron, agraphite contained in the cast iron is melted and functions as alubricant which reduces a rise of the friction coefficient at the hightemperature, thereby reducing the amount of wear of the frictionsurface. In the present flywheel formed of a carbon steel material, thefriction surface the temperature of which tends to rise, is desirablysubjected to shot peening to form a matrix of minute recesses, which iseffective to prevent an excessive rise of the temperature at thefriction surface and an excessive increase of the friction coefficientof the friction surface.

It is noted that an excessive rise of the friction coefficient of thefriction surface of the flywheel may cause a quick engagement of thefriction member with the friction member such as a clutch disc. In thiscase, the flywheel when used for an engine of an automobile may sufferfrom the following problems:

a) The clutch of the vehicle cannot be smoothly operated, leading toeasy stall of the engine.

b) The power transmission system of the vehicle may be subject to anabrupt change in the load torque acting thereon, leading to reducedservice life of the power transmission system.

c) The engine of the vehicle is likely to suffer from a judderingphenomenon at a relatively low operating speed.

d) The clutch disc suffers from an excessive wear, leading to reducedservice life thereof.

According to a further preferred form of the flywheel of this invention,the friction surface is hardened prior to the shot peening. Forinstance, the friction surface is hardened to within a range of aboutHRC45-47. The hardening of the friction surface is effective to reducethe rate of wearing of the friction surface. The wearing of the frictionsurface causes an increase in the smoothness (i.e., a decrease in theroughness) of the friction surface, even where the friction surface isinitially given the minute recesses. In other words, the hardening ofthe friction surface maintains the minute recesses in their originalshape and size for a prolonged period of time, and thereby maintains asufficient amount of air between the friction surface and the frictionmember, to thereby hold the friction coefficient therebetween at a valuelow enough to prevent an excessive rise of the temperature, which wouldcause thermal decomposition of the material of the clutch disc.Therefore, the hardening of the friction surface permits the presentcarbon steel flywheel to have operating durability and service lifewhich are comparable with those of the conventional gray cast ironflywheel.

The second object indicated above may be achieved according to a secondaspect of this invention, which provides a process of manufacturing aflywheel which is generally disc-shaped and which includes a ring geardisposed along a periphery thereof and has a friction surface on one ofaxially opposite sides thereof, characterized by comprising the stepsof: (a) forging a carbon steel blank to obtain an intermediate productwhich is a generally disc-shaped integral one-piece forged structureconsisting of an inner portion having the friction surface and an outerring gear portion which serves as the ring gear and which is disposedradially outwardly of the inner portion; (b) hardening the frictionsurface of the inner portion of the intermediate product; and (c) shotpeening the hardened friction surface to form a multiplicity of minuterecesses in the hardened friction surface.

The process of the present invention has the same advantages asdescribed above with respect to the flywheel of the invention, and theadvantages described above with respect to the minute recesses formed inthe friction surface and the hardening of the friction surface.

The principle of the present invention is particularly applicable to aflywheel which is coaxially connected to the crankshaft of an engine ofa motor vehicle and which is a generally disc-shaped member having anouter ring gear portion. However, the principle of the invention isequally applicable to any other rotating member. Where the flywheel isused with the engine crankshaft, a pinion driven by an engine startermotor meshes with the outer ring gear portion of the flywheel.

According to one preferred form of the present process, the step offorging a carbon steel blank to obtain an intermediate productcomprises: hot-forging a cylindrical carbon steel blank to obtain afirst cylindrical intermediate product having an inner disc portion andan outer annular portion which are integral with each other; andcold-forging the first cylindrical intermediate product to form theouter ring gear portion at the outer annular portion, for therebyobtaining a second cylindrical intermediate product as the generallydisc-shaped integral one-piece forged structure consisting of the innerportion and the outer ring gear portion.

However, the generally disc-shaped integral one-piece forged structureof the flywheel may be obtained in a single forging step.

In one arrangement of the above preferred form of the present process,the step of cold-forging the first cylindrical intermediate productcomprises moving the first cylindrical intermediate product relative toa gear forming die in an axial direction of the first cylindricalintermediate product, the gear forming die having a multiplicity oftooth forming teeth for forming a multiplicity of teeth of the outerring gear portion.

The process according to another preferred form of this inventionfurther comprises the step of hardening a surface of the outer ring gearportion. The process may further comprise the step of cutting aplurality of holes through the attaching section of the inner portion,which holes are used for attaching the flywheel to the crankshaft orother rotary member.

The shot peening of the friction surface may be effected with steelparticles driven by a blast of compressed air or by a centrifugal forceagainst the friction surface, at a velocity of about 100 m/sec, forexample. The steel particles have a diameter or size selected within arange of 40-200 μm and a hardness not lower than that of the carbonsteel material of the flywheel (blank). The minute recesses or microholes formed in the hardened friction surface preferably have a depth ofseveral microns. The shot peening is preferably adapted to give thefriction surface a hardened layer having a thickness of about 10-20 μm,which is larger than the depth of the minute recesses. The hardness ofthe hardened layer is higher than the hardness given by the hardeningoperation such as induction hardening. The shot peening may be referredto as “wide peening cleaning (WPC)”.

The step of hardening the friction surface is preferably effected byinduction hardening such as hardening by high-frequency inductionheating, prior to the shot peening of the friction surface. The frictionsurface may be hardened to HRC45-47.

BRIEF DESCRIPTION OF DRAWINGS

The above and optional objects, features, advantages and technical andindustrial significance of this invention will be better understood byreading the following detailed description of the invention, whenconsidered in connection with the accompanying drawings, in which:

FIG. 1 is a side elevational view in cross section of a flywheel for anautomobile constructed according to one embodiment of the presentinvention, taken along line 1—1 of FIG. 2;

FIG. 2 is a front elevational view of the flywheel of FIG. 1;

FIG. 3 is a schematic view illustrating a flow of two process steps formanufacturing the flywheel of FIG. 1;

FIG. 4 is a view indicating a first forging step in the process of FIG.3;

FIG. 5 is a view indicating a second forging step in the process of FIG.3;

FIG. 6(a) is a view showing micro holes formed by shot peening in thefriction surface of the flywheel of FIG. 1;

FIG. 6(b) is a view showing a friction surface of the conventionalflywheel formed by casting of a gray iron; and

FIG. 7 is a view for explaining a process of manufacturing aconventional flywheel.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring first to FIGS. 1 and 2, there is shown a flywheel 10constructed according to one embodiment of this invention. The flywheel10 is used with a crankshaft of an internal combustion engine of anautomotive vehicle. The flywheel 10 is a generally disc-shaped structureconsisting of a cylindrical inner portion 14 and an annular outer ringgear portion 16 disposed radially outwardly of the inner portion 14. Theinner portion includes a central attaching section 11 which has sixholes 12 formed therethrough. The flywheel 10 is attached at itsattaching section 12 to the crankshaft of the engine, with bolts beinginserted through the holes 12, such that the flywheel 10 is coaxial withthe crankshaft.

As shown in FIG. 1, the central attaching section 11 of the innerportion 14 has a comparatively small wall thickness or axial dimension,while the outer ring gear portion 16 has a comparatively large wallthickness or axial dimension. The outer ring gear portion 16 locatedradially outwardly of the inner portion 14 is an annular portionextending from the inner portion 14 in the axial direction. The outerring gear portion 16 consists of a toothed section 16 a and a basesection 16 b which are two mutually adjacent axial half sections, twosubstantially equal sections which are mutually adjacent to each otherin the axial direction of the flywheel 10. The toothed section 16 a,which is located on the right-hand side as seen in FIG. 1, has amultiplicity of grooves 17 which are evenly spaced apart from each otherin the circumferential direction of the annular outer ring gear portion16, as shown in FIG. 2. The grooves 17 are open in the outercircumferential surface of the toothed section 16 a and in the annularend face of the toothed section 16 a remote from the base section 16 b,as shown in FIGS. 1 and 2. In the presence of the multiple grooves 17,the toothed section 16 a has a multiplicity of teeth 18 which project inthe radially outward direction of the flywheel 10, as shown in FIG. 2,and extend in the axial direction of the flywheel 10. The teeth 18 arespaced apart from each other in the circumferential direction of theouter ring gear portion 16, as shown in FIG. 2. The teeth 18 areconnected to each other by the base portion 16 b adjacent to the toothedsection 16 a, as well as by the radially inner circumferential part ofthe toothed section 16 a.

Since the outer ring gear portion 16 has a sufficiently large wallthickness or axial dimension, the flywheel 10 has a sufficiently highratio of the weight of the outer ring gear portion 16 to the totalweight of the flywheel 10, and therefore has a sufficiently largemomment of inertia, without increasing the diameter of the flywheel 10.

The inner portion 14 has an annular friction surface 19 slightlyprojecting in the axial direction opposite to the direction in which theannular outer ring gear portion 16 extends. Namely, the friction surface19 is provided on one of the opposite surfaces of the inner portion 14,which is remote from the toothed section 16 a of the outer ring gearportion 16. The friction surface 19 is provided for frictional contactwith a clutch disc under pressure. The friction surface 19 lies in aplane perpendicular to the axis of rotation of the flywheel 10.

The flywheel 10 is manufactured in three major process steps, asindicated in FIG. 3. In the first step, a carbon steel blank 24 ofcylindrical shape, i.e. a cylindrical blank, of a carbon steel materialfor machine structural use, for instance, carbon steel S35C (JIS), ishot-forged into a first cylindrical intermediate product 26, between anupper die 20 and a lower die 22, as shown in FIG. 4. The upper die 20has a forming surface substantially following the shape of the left-handside surface (as seen in FIG. 1) of the flywheel 10, which surfaceincludes the friction surface 19. On the other hand, the lower die 20has a forming surface substantially following the shape of theright-hand side surface of the flywheel 10. With the carbon steel blank24 placed in the lower die 22, the upper and lower dies 20, 22 are movedtoward each other, whereby the blank 24 is formed into the firstcylindrical intermediate product 28 which has an inner disc portion 27corresponding to the inner portion 14 of the flywheel 10, and an annularportion 28 which is radially outward of and integral with the inner discportion 27. The first cylindrical intermediate product 28 is almostsimilar in shape to the flywheel 10, but does not have the teeth 18.That is, the toothed section 16 a has not been formed on the annularportion 28 of the first intermediate product 28. The annular portion 28has an outside diameter substantially equal to that of the flywheel 10(outer ring gear portion 16), and an axial dimension which is slightlysmaller than that of the outer ring gear portion 16 of the flywheel 10to be manufactured.

In the second step, the first cylindrical intermediate product 26 iscold-forged using an upper die 31, a lower die 32, and a gear formingdie 34, as shown in FIG. 5. The upper and lower dies 31, 32 cooperate todefine a cavity substantially following the shape of the firstcylindrical intermediate product 26. The gear forming die 34 has amultiplicity of tooth forming teeth 30 which are spaced apart from eachother at the same pitch as the grooves 17 of the toothed section 16 aand each of which has a shape substantially identical with the grooves17. With the first intermediate product 26 held between the upper andlower dies 31, 32, the first intermediate product 26 and the gearforming die 34 are moved toward each other in the axial direction of theproduct 26, so that the grooves 17 are formed by flows of the materialof the product 26 by the respective tooth forming teeth 30 of the gearforming die 34, whereby the multiplicity of teeth 18 are formed. Thematerial of the first intermediate product 2 which has been moved by thetooth forming teeth 30 to form the grooves 17 is forced to flow towardthe end of the annular portion 28. Thus, the first cylindricalintermediate product 26 is cold-forged into a second cylindricalintermediate product 38 which includes the outer ring gear portion 16having the desired axial dimension. In other words, the axial dimensionof the annular portion 28 is increased to the nominal value of the outerring gear portion 16, by cold-forging the first cylindrical intermediateproduct 26.

In the third step, the second cylindrical intermediate product 28 issubjected to a drilling operation to form the six holes 12 through thecentral portion of the intermediate product 28, and an inductionhardening operation to harden the surface of the outer ring gear portion16. The second intermediate product 28 is further subjected to aninduction hardening operation to harden the friction surface 19 to ahardness value within a range of about HRC45-47, so that the durabilityof the friction surface 19 is improved.. Then, the hardened frictionsurface 19 is subjected to a shot peening operation using steelparticles driven by a blast of compressed air or by a centrifugal forceagainst the friction surface 19, at a velocity of about 100 m/sec. Thesteel particles have a diameter or size selected within a range of40-200 μm. As a result, a multiplicity of micro holes or minute recesses42 are formed in the hardened friction surface 19, as illustrated in anenlarged view of FIG. 6(a). The micro holes 42 have a depth of aboutseveral microns. The shot peening gives the initially hardened frictionsurface 19 a hardened layer having a thickness of about 20 μm, which isconsiderably larger than the depth of the micro holes 42. The hardenedlayer has a hardness of about HRC70-90. FIG. 6(b) illustrates a frictionsurface of the conventional flywheel of a gray cast iron.

The flywheel 10 according to the present embodiment constructed asdescribed above is an integral one-piece forged structure formed byforging the carbon steel blank 24 for machine structural use.Accordingly, the tensile strength of the flywheel 10 at the attachingsection 11 of its inner portion 14 is significantly increased, so thatthe required wall thickness at the attaching section 11 can beaccordingly reduced. Therefore, the present flywheel 10 may be designedto have an increased ratio of the weight of the outer ring gear portion16 to the total weight, and an accordingly increased moment of inertia.Accordingly, the amount of variation of the operating speed of thecrankshaft is reduced, and the fuel economy of the engine is improved.

Further, the outer ring gear portion 16 formed by forging has a higherstrength owing to a continuous metal structure, than the ring gear ofthe conventional flywheel, which is formed by hobbing or any other toothcutting operation. In addition, the present one-piece flywheel 10 can bemanufactured at a reduced cost, with a reduced number of process steps,since the integral one-piece forging structure of the flywheel 10 isformed by forcing of the carbon steel blank 24, without having toprepare two separate members, that is, an inner body member and a ringgear member, and assemble these two members together, as required tomanufacture the conventional flywheel.

The outer ring gear portion 16 consists of the toothed section 16 a andthe base section 16 b which are two mutually adjacent axial halfsections of the outer ring gear portion. Since the teeth 18 of thetoothed section 16 a are connected to each other by the base section 16b which is adjacent to the toothed section 16 a in the axial directionof the outer ring gear portion 16, the toothed section 16 a has anincreased mechanical strength, in particular, increased toughness, evenwhere the toothed section 16 a is hardened. Accordingly, the flywheel 10exhibits improved durability at a high operating speed.

The present flywheel 10 has a further advantage which is derived fromthe multiplicity of micro holes 42 formed in the friction surface 19 byshot peening. In operation of the flywheel 10, the friction surface 19is brought into frictional contact with a friction member such as aclutch disc. In the presence of the micro holes 42 formed in thefriction surface 19 of the flywheel 10, there remains a network ormatrix of air between the friction surface 19 and the friction member,which makes it possible to minimize an increase of the frictioncoefficient of the friction surface 19 even at a relatively highoperating temperature, thereby reducing by the amount of wear of thefriction surface 19 and prolonging the expected service life of thefriction surface 19. In this respect, it is noted that the frictionalsliding contact of the friction surface 19 and the friction membercauses a rise of the temperature at the friction surface 19. In theconventional flywheel including the inner body member formed of a castiron, a graphite contained in the cast iron is melted and functions as alubricant which reduces a rise of the friction coefficient at the hightemperature, thereby reducing the amount of wear of the frictionsurface. In the present flywheel 10 formed of a carbon steel material,the friction surface 19 the temperature of which tends to rise issubjected to shot peening to form a matrix of micro holes 42, which iseffective to prevent an excessive rise of the temperature at thefriction surface 19 and an excessive increase of the frictioncoefficient of the friction surface 19.

Further, the hardening of the friction surface 19 is effective to reducethe rate of wearing of the friction surface 19. The wearing of thefriction surface 19 causes an increase in the smoothness (i.e., adecrease in the roughness) of the friction surface 19, even where thefriction surface is initially given the micro holes 42. In other words,the hardening of the friction surface 19 maintains the micro holes 42 intheir original shape and size for a prolonged period of time, andthereby maintains a sufficient amount of air between the frictionsurface 19 and the friction member, to thereby hold the frictioncoefficient therebetween at a value low enough to prevent an excessiverise of the temperature, which would cause thermal decomposition of thematerial of the clutch disc. Therefore, the hardening of the frictionsurface permits 19 the present carbon steel flywheel to have operatingdurability and service life which are comparable with those of theconventional flywheel formed of a gray cast iron.

While the presently preferred embodiment of this invention has beendescribed above in detail with a certain degree of particularity, byreference to the accompanying drawings, it is to be understood that thepresent invention is not limited to the details of the illustratedembodiment, but may be otherwise embodied.

For instance, the friction surface 19 need not be subjected to aninduction hardening operation, and may be subjected to a shot peeningoperation without prior induction hardening.

While the friction surface 19 is hardened by high-frequency inductionheating, it may be hardened by any other hardening treatment.

It is to be understood that the present invention may be embodied withvarious other changes, modifications and improvements, which may occurto those skilled in the art, without departing from the spirit and scopeof the invention defined in the following claims.

What is claimed is:
 1. A flywheel which is disc-shaped for engaging afriction member, said flywheel including a ring gear disposed along aperiphery thereof and having a friction surface of axially oppositesides thereof, for frictional contact with said friction member whereinan improvement comprises: said flywheel being an integral one-pieceforged structure which is formed by forging a carbon steel blank andwhich consists of an inner portion having said friction surface and anouter ring gear portion which serves as said ring gear and which isdisposed radially outwardly of said inner portion; and said frictionsurface having a multiplicity of minute recesses formed by shot peeningafter said forging on said carbon steel blank.
 2. A flywheel according to claim 1, wherein said carbon steel blank is a cylindrical blank andsaid outer ring gear portion comprises a toothed section and a basesection which are two mutually adjacent axial sections of said outerring gear portion, said toothed section being formed by forging saidcylindrical blank in an axial direction thereof, said toothed sectionhaving a multiplicity of teeth which project in a radial outwarddirection of said outer ring gear portion and extend in an axialdirection of said outer ring gear portion and which are spaced apartfrom each other in a circumferential direction of said outer ring gearportion, said multiplicity of teeth being connected to each other bysaid base section.
 3. A flywheel according to claim 2, wherein saidtoothed section having said multiplicity of teeth is located on theother of said axially opposite sides of said flywheel, which is remotefrom said friction surface.
 4. A flywheel according to claim 1, whereinsaid friction surface is hardened prior to said shot peening.
 5. Aflywheel according to claim 1, wherein said said carbon steel blank isformed of a carbon steel material for machine structural use.
 6. Aprocess of manufacturing a flywheel which is disc-shaped and whichincludes a ring gear disposed along a periphery thereof and has afriction surface on one of axially opposite sides thereof, comprisingthe steps of: forging a carbon steel blank to obtain an intermediateproduct which is a disc-shaped integral one-piece forged structurecomprising additional an inner portion having said friction surface andan outer ring gear portion which serves as said ring gear and which isdisposed radially outwardly of said inner portion; hardening saidfriction surface of said inner portion of said intermediate product; andshot peening the hardened friction surface to form a multiplicity ofminute recesses in said hardened friction surface.
 7. A processaccording to claim 6, wherein said step of forging a carbon steel blankto obtain an intermediate product comprises: hot-forging a cylindricalcarbon steel blank to obtain a first cylindrical intermediate producthaving an inner disc portion and an outer annular portion which areintegral with each other; and cold-forging said first cylindricalintermediate product to form said outer ring gear portion at said outerannular portion, for thereby obtaining a second cylindrical intermediateproduct as said disc-shaped integral one-piece forged structureconsisting of said inner portion and said outer ring gear portion.
 8. Aprocess according to claim 7, wherein said step of cold-forging saidfirst cylindrical intermediate product comprises moving said firstcylindrical intermediate product relative to a gear forming die in anaxial direction of said first cylindrical intermediate product, saidgear forming die having a multiplicity of tooth forming teeth forforming a multiplicity of teeth of said outer ring gear portion.
 9. Aprocess according to claim 6, further comprising the step of hardening asurface of said outer ring gear portion.
 10. A process according toclaim 6, wherein said carbon steel blank is formed of a carbon steelmaterial for machine structural use.
 11. A flywheel which is disc-shapedand which includes an inner portion having a friction surface on one ofaxially opposite sides thereof, and a ring gear disposed radiallyoutwardly of said inner portion, wherein said friction surface has amultiplicity of minute recesses formed by shot peening.
 12. A flywheelaccording to claim 11, which is an integral one-piece forged structurewhich is formed by forging a carbon steel blank and which consists ofsaid inner portion having said friction surface and an outer ring gearportion which serves as said ring gear.
 13. A flywheel according toclaim 11, wherein said multiplicity of minute recesses are formed byshot peening after said friction surface is hardened.